1. Field of the Invention
The invention relates to a secondary air supply system and an abnormality detection method for a secondary air supply system. More specifically, the invention relates to a secondary air supply system which supplies secondary air to each exhaust passage of an internal combustion engine that includes an independent exhaust passage for each cylinder group, as well as an abnormality detection method for a secondary air supply system.
2. Description of the Related Art
An internal combustion engine mounted in a vehicle such as a passenger car or a truck is typically provided with an exhaust gas control apparatus in an exhaust passage of the engine. This exhaust gas control apparatus has a three way catalyst or the like which reduces noxious substances such as CO, HC, and NOx in the exhaust gas. The performance with which this exhaust gas control apparatus purifies these kinds of noxious substances in the exhaust gas improves as the operating temperature rises. Therefore, when the operating temperature is low such as during a cold start of the internal combustion engine, the exhaust gas control apparatus may not be able to sufficiently reduce the noxious substances in the exhaust gas.
Therefore, conventional engines are provided with a secondary air supply system which supplies an oxidizing gas, in this case secondary air, to the upstream side of the exhaust gas control apparatus in the exhaust passage. Supplying secondary air to the exhaust pipe on the upstream side of the exhaust gas control apparatus during a cold start of the internal combustion engine, for example, using this secondary air supply system increases the oxygen concentration of the exhaust gas passing through that exhaust pipe. This increased oxygen concentration promotes the oxidation of noxious substances (in this case CO and HC) in the exhaust gas, thereby reducing the noxious substances in the exhaust gas.
If there is an abnormality in the secondary air supply system, e.g., if a secondary air supply line that introduces the secondary air into the exhaust pipe becomes clogged or a secondary air supply pump that supplies the secondary air to the exhaust pipe via this secondary air supply line fails, for example, the air flow rate of the secondary air supplied to the exhaust passage decreases. As a result, the noxious substances in the exhaust gas are not suppressed without oxidation of noxious substances (in this case CO and HC) in the exhaust gas being promoted, which may result in a deterioration of emissions. Therefore, technology for detecting an abnormality in a secondary supply system in a conventional internal combustion engine has been proposed.
Japanese Patent Application Publication No. JP-A-2003-83048, for example, describes technology in which a pressure sensor is arranged in a secondary air supply passage which supplies secondary air. This technology detects an abnormality in the secondary air supply system based on a pressure value detected by this pressure sensor and a pressure fluctuation value.
Some internal combustion engines such as horizontally-opposed engine or V-type engines have a plurality of cylinder groups made up of at least one or more cylinders. Some of these internal combustion engines having such a plurality of cylinder groups have an independent exhaust passage provided for each group of cylinders. In internal combustion engines having an independent exhaust passage provided for each group of cylinders, the secondary air supply system described above supplies secondary air to each of the exhaust passages on the upstream side of the exhaust gas control apparatuses provided in the exhaust passages.
FIG. 8 is a graph showing the relationship between the secondary air flow rate and the amount of noxious substances. As shown in the drawing, the secondary air flow rate supplied to the exhaust passage and the amount of noxious substances in the exhaust gas discharged outside of the internal combustion engine from the exhaust passage are not proportional to one another. Rather, the amount of noxious substances noticeably increases in a quadratic curve as the secondary air flow rate decreases. Accordingly, in a secondary air supply system provided in a conventional engine having a plurality of cylinder groups, even if the total air flow rate of the combined secondary air flow rates supplied to the exhaust passages is the same, if the percentages of the secondary air flow rates supplied to the exhaust passages are different, it will cause the total amount of noxious substances, which is the combined amount of the noxious substances in the exhaust gas discharged outside of the internal combustion engine from the exhaust passages, to change. That is, if an abnormality in the secondary air supply system is detected based only on the total air flow rate of the secondary air in a conventional internal combustion engine having a plurality of cylinder groups, that detection accuracy will be low.